It is known that biphenyl compounds can be produced by oxidative coupling an aromatic compound in an oxygen-containing atmosphere in the presence of a palladium type catalyst by various methods. For example, Japanese patent application Publication (Kokoku) No. 48--1054(1973) discloses a method of producing a biphenyl compound by dehydrogenation-dimerizing (oxidative coupling) a benzene type aromatic compound in an oxygen-containing atmosphere in the presence of an organic palladium salt but in the absence of reaction medium under an increased pressure.
Also, it is known that a conventional oxidative coupling method, for example, the above-mentioned method, for the aromatic compounds, is applied to an orthophthalic ester, the resultant coupling product contains, as major components, 2,3,3',4'-biphenyltetracarboxylic tetraester (a-BPTT) and 3,3',4,4'-biphenyltetracarboxylic tetraester (S-BPTT), and usually, the amount of the a-BPTT is larger than that of the S-BPTT. However, the S-BPTT is useful as a material for producing an aromatic carboxylic dianhydride which is useful as an intermediate for producing an aromatic polyimide resin having excellent tenacity, thermal resistance and electric insulating property. Therefore, the S-BPTT is more industrially valuable than the a-BPTT. Accordingly, it is desirable to provide a new method of oxidative coupling the orthophthalic ester, which method is capable of producing the S-BPTT as a main product with a high degree of yield thereof and the a-BPTT as a by-product with a very low degree of yield thereof. It is preferable that the yield of the S-BPTT be as large as possible and the yield of the a-BPTT be as small as possible.